1. Field of the Invention
The present invention relates to an image display device and a video signal processing method used in same and, more specifically to, an image display device and a video signal processing method used in the same that are well suited for applications in a case where a video signal is transmitted through a video signal line to a data driver by utilizing a differential transmission system, for example, in the case of a liquid crystal display (LCD) and a plasma display device.
2. Description of the Related Art
The thin image display devices such as an LCD and a plasma display device have come to have a longer transmission distance of video signals therein owing to an increasing screen size in the recent years and also to have a larger amount of data transmitted by the video signal owing to an increasing resolution of a display panel, so that the image display devices are required to have more wirings and higher transmission rates. In such a case, the image display devices suffer from deterioration in electro-magnetic interference (EMI) properties and severer conditions for accurate transmission of the video signal and, on the other hand, need to save on power and space, specifically, on power consumed in internal circuitry and realize miniaturization and higher-density packaging of the circuit components. It has caused the recent image display devices to mainly employ a video signal transmission method referred to as the differential transmission system, by which the number of the required video signal lines will be reduced.
Further, this type of the image display device may receive a variety of video signals incoming thereto; therefore, on its substrate for transmitting the video signals directed to a display panel, if the video signals transmitted through mutually neighboring wirings and so susceptible to wiring crosstalk, such as those with mutually reversedpolarities (for example, polarities “1” and “0”), are received, effective voltage amplitudes of the video signals are reduced, and if the effective voltage amplitudes become less than an input amplitude specification to be used as a minimal reference amplitude below which a data driver cannot operate properly, display noise such as flicker occurs on a screen. To avoid such a situation, if a voltage amplitude output from a timing controller is set large beforehand so that the effective voltage amplitudes may be sufficiently large as compared to the input amplitude specification value for the data driver, dissipation power increases along with another bad effect of tradeoff in deterioration of the EMI properties. On the other hand, if inter-wiring spacing or a ground wiring is to be provided which is long enough to avoid the signals from interfering with each other even when exposed to wiring crosstalk, the substrate needs to be larger in area, leading to a bad effect of not only preventing miniaturization of the device as a whole but also increasing the costs.
As this kind of related art, there is provided an image display device shown in FIG. 7.
As shown in FIG. 7, the image display device includes a display panel 1, a scan driver 2, a data driver 3, a timing controller 4, and a video signal line 5. The timing controller 4 is fitted with a data signal output section 4a. The display panel 1 includes a liquid crystal display (LCD) and has scanning lines along predetermined rows as well as data lines 1b along predetermined columns and pixels (not shown) each of which is positioned at an intersection of each of the scanning lines is and each of the data lines 1b. The data driver 3 drives the data lines 1b on the display panel 1. Based on a control signal ct1 received from the timing controller 4, the data driver 3 writes pixel data based on a supplied data signal vj to each of the data lines 1b. 
Based on a control signal ct2 received from the timing controller 4, the scan driver 2 outputs a scanning line drive signal intended to drive the scanning lines 1a on the display panel in predetermined order (for example, in a line sequence). The timing controller 4 generates an input signal receivable by the data driver 3 based on an externally input video signal vi, provides the data driver 3 with the control signal ct1, sets a voltage amplitude of the input signal, and sends the signal as the data signal vj from the data signal output section 4a through a video signal line 5, while providing the scan driver 2 with the control signal ct2. The video signal line (data signal transmission line) 5 is used to send the data signal vj by utilizing a differential transmission system and has such a number of signal lines as to be needed when a binary representation of the maximum value of a gradation level of at least the video signal vi is applied to a reduced swing differential signaling (RSDS: one digital interface technology for use in LCD panels) transmission format, so that each of the mutually neighboring pairs of those signal lines may send the differential transmission system-complying data signals vj having the mutually reverse phases, as one pair of signal lines.
FIG. 8 is an explanatory diagram of a transmission format for an RSDS signal.
As shown in FIG. 8, the transmission format of, for example, an eight-bit RSDS signal is made of four pairs of transmission signals in which a total of eight transmission signals are arranged because each of the four pairs include two differential signals of positive-polarity and negative-polarity ones, so that data may be latched at the trailing edge and the leading edge of a transmission clock signal CLK. If the transmission data signal vj is of, for example, 198 gradations, the 198-th gradation level is represented as “11000110” in binary number and has an actual waveform of “HHLLLHHL” in a bit string. If applied to the RSDS transmission format, the bit string may be given as shown in FIG. 8. It is to be noted a transmission signal D (0) indicates a least significant bit (LSB) and a transmission signal ID (7) indicates a most significant bit (MSB). If the video signal vi in FIG. 7 described above is, for example of eight bits, the maximum is the 255-th gradation level and represented as “11111111” in binary number, so that if applied to the RSDS transmission format, the maximum level requires four pairs of video signals because data is latched at the two edges of each pulse of the clock signal; further because the video signals are of the differential transmission system, a total of eight video signal lines 5 (=four positive-polarity signal lines 5 and four negative-polarity signal lines 5) will be required. It is to be noted that even in the case of any other differential transmission systems such as mini-LVDS, similarly, the number of the signal lines required is determined by applying the bit string to the transmission format.
In the image display device in FIG. 7, the video signal vi is input to the timing controller 4, where the vide signal vi is rearranged into a signal receivable by the data driver 3; then the data signal output section 4a in the timing controller 4 sets an amplitude of a transmission voltage of the signal and outputs it as the data signal vj, being accompanied by the generation of the corresponding generation timing signal, the horizontal reference signal (control signal ct1) directed to the data driver 3, and the vertical reference signal (control signal ct2) directed to the scan driver 2. The data signal vj whose transmission voltage amplitude is set in such a manner is sent to the data driver 3 via the video signal lines 5 that comply with the differential transmission system. Then, an image that corresponds to the video signal vj will be displayed on the panel 1.
It is to be noted that the video signal vi is rearranged by the timing controller 4 into a data signal vj intended to drive the data driver 3, in which case the data signal vj is transmitted through the video signal line 5 in accordance with a predetermined transmission format. The transmission format corresponds to, for example, an eight-bit RSDS signal. The data signal vj corresponding to the RSDS signal is a digital signal (whose high and low levels are indicated by “H” and “L” respectively, which are in turn indicated by “1” and “0” respectively) at the same time as being a differential signal. Two of the video signal lines 5 through which the differential data signals vj are transmitted are paired: one for the positive-polarity video signal and the other for the negative-polarity one. Those video signals come in the “H” signal if their respective polarities' potentials are higher than the reference voltage (potential) of the data signal vj and “L” signal if those potentials are lower than reference voltage (potential). Further, if a remainder is positive which is obtained by subtracting a potential of the negative polarity video signal from a potential of the positive-polarity video signal, those signals come in an “H” level differential signal; on the other hand, if the remainder is negative which is obtained by subtracting the potential of the negative polarity video signal from the potential of the positive-polarity video signal, those signals come in an “L” level differential signal. Additionally, in the differential transmission system, the clock signal for transmission of the data signal vj is also a differential signal.
FIG. 9 is a chart for showing an example of waveforms of the data signal vj transmitted through the video signal lines 5.
The RSDS signals are a differential signal and, as shown in FIG. 9, their polarity is determined to be “H” or “L” by a pair of a video signal DATA (+) along the positive-polarity video signal line and a video signal DATA (−) along the negative-polarity video signal line. For example, if the video signal has a reference potential of 1.1V and the video signals DATA (+) and DATA (−) have potentials of 1.2V and 1.0V respectively, the differential signal has a value of +200 mV and so comes in the “H” level. On the other hand, if the video signal has a reference potential of 1.1V and the video signals DATA (+) and DATA (−) have potentials of 1.0V and 1.2V respectively, the differential signal has a value of −200 mV and so comes in the “L” level. To recognize the “H” level in the data driver 3, the value of +200 mV of the differential signal needs to be higher than the “H” level threshold in the data driver 3; similarly, to recognize the “L” level, the value of −200 mV needs to be lower than the “L” level threshold in the data driver 3.
If the input video signal vi is of, for example, 179 gradations, the digital signal has a bit string of “HLHHLLHH” (“10110011”), which is arranged in accordance with the RSDS signal transmission format as shown in Gradation pattern [1] in FIG. 9. Further, if the input video signal vi is of, for example, 140 gradations, the digital signal has a bit string of “HLLLHHLL” (“10001100”), which is arranged in accordance with the RSDS transmission format as shown in Gradation pattern [2] in FIG. 9.
FIGS. 10 and 11 are a chart for showing an influence from wiring crosstalk received from the data signal vj transmitted through the neighboring video signal lines.
FIGS. 10 and 11 show how the amplitude of a voltage transmitted through one of the video signal lines is affected by a digital signal transmitted through its neighboring video signal line. The digital signal has shown pattern of “H” and “L”. That is, FIG. 10 shows the influence on the DATA (+) signal of interest in the first pair extracted along with the second pair from the chart of the waveforms of the data signal vj in FIG. 9. On the other hand, FIG. 11 shows an example where the data signal vj has a different gradation pattern from that in FIG. 10. It is to be noted that in the Neighbor pattern [1] in FIG. 10, the video signal DATA (+) in the first pair of the video signal lines of interest is neighbored by the video signal DATA (−) which is in the second pair and has the “H” polarity, the same as that of the video signal DATA (+) having the “H” polarity. Further, similarly, in the Neighbor pattern [2], the video signal DATA (+) along the video signal line of interest has the “L” polarity, which is the same polarity as that of the signal DATA (+) in the second pair having the “L” polarity.
On the other hand, in the Neighbor pattern [3] in FIG. 11, the video signal DATA (+) along the video signal line of interest has the “H” polarity, whereas the video signal DATA (−) in the neighboring second pair has the “L” polarity, which is the reverse polarity. Similarly, in the Neighbor pattern [4] also, the video signal DATA (+) along the video signal line of interest has the “L” polarity, whereas the video signal DATA (−) in the neighboring second pair has the “H” polarity, which is the reverse polarity. In such a manner, in the case of differential signals, the video signals transmitted through the mutually neighboring two video signal lines have the four polarity patterns. That is, there are those four neighbor patterns because the differential video signals DATA (+) and DATA (−) in each of the pairs have the mutually reverse polarities always.
When the signals having those four neighbor patterns respectively are being transmitted through the video signal lines 5, a transmission voltage amplitude of the video signal DATA (+) transmitted through the video signal line of interest changes as it is affected by its neighbor patterns.
Hereinafter, in explanation, the amplitude of a voltage of the video signal as output from the timing controller 4 is referred to as “output voltage amplitude” and that of the video signal actually being transmitted through the video signal line 5 is referred to as “effective voltage amplitude”.
In the Neighbor patterns [1] and [2] in FIG. 10, the video signal along the video signal line of interest has the same polarity as the video signal along the video signal line in the neighboring pair, so that those two video signals have little difference in potential; therefore, the effective voltage amplitude of the video signal along the video signal line of interest is not affected by the video signal line in the neighboring pair. Accordingly, its output voltage amplitude and effective voltage amplitude are almost the same as each other.
On the other hand, in the Neighbor patterns [3] and [4] in FIG. 11, the video signals along the paired video signal lines that neighbor the video signal line of interest have the mutually reverse polarities and so have a difference in potential therebetween, which difference interferes with the video signal along the video signal line of interest, that is, the signal DATA (+) in the first pair and the signal DATA (−) in the second pair, so that their effective voltage amplitudes become smaller than the output voltage amplitude. In such a manner, the effective voltage amplitude of the video signal transmitted through a given video signal line may become smaller than the output voltage amplitude owing to influence of the video signal transmitted through the neighboring video signal line, which phenomenon may be the wiring crosstalk.
The image display device shown in FIG. 7 has a problem in that display noise such as flicker occurs on a screen, because a transmission error occurs if the effective voltage amplitude of the data signal vj transmitted via the video signal line 5 becomes smaller than the input amplitude specification value for the data driver 4 owing to an influence from wiring crosstalk. For example, in the Neighbor patterns [3] and [4] in FIG. 11, due to the influence from wiring crosstalk, the effective voltage amplitude becomes smaller than the output voltage amplitude, so that there is a possibility that display noise may occur. The degree of the influence from the wiring crosstalk depends on the gradation in the incoming video signal vi; for example, the wiring crosstalk from the neighboring pair has no influence if the video signal vi is input which is of such a gradation that the video signal along the video signal line of interest may have the same polarity as that of the video signal along the video signal line in the neighboring pair as shown in FIG. 10.
In such an image display device that countermeasures are taken on the problem, an output voltage amplitude is set large by the timing controller beforehand so that even if a given video signal line is affected by wiring crosstalk from the neighboring video signal line, an effective voltage amplitude of the corresponding video signal may not become lower than an input amplitude specification value for the data driver. However, the output voltage amplitude is set large beforehand, so that a new problem occurs in an increase in dissipation power. That is, in a case where wiring crosstalk has a large degree of an influence so that an effective voltage amplitude may be decreased, when an output voltage amplitude is set beforehand so that the effective voltage amplitude may become slightly greater than an input amplitude specification value for the data driver and if a video signal is input which may be less affected by the neighboring video signal line as in the case of the Gradation pattern [1] or [2] in FIG. 10, for example, the effective voltage amplitude increases and exceeds the input amplitude specification value for the data driver more than necessary, thereby dissipating extra power. Further, in such a case, there occurs a problem in that EMI may increase due to the effective voltage amplitude in excess of the input amplitude specification value for the data driver more than necessary.
FIG. 12 is a block diagram for showing an electrical configuration of important components of the image display device that countermeasures are taken on the problem.
As shown in FIG. 12, the image display device includes data drivers 131, 132, . . . , 13M, and 13M+1, a timing controller 14, and video signal lines 15; it further includes a scan driver and a display panel which are not shown but similar to the scan driver 2 and the display panel 1 in FIG. 7 respectively. The video signal line is arranged similar to the video signal line 5 in FIG. 7. The timing controller 14 has a data signal output section 14a, a video signal processing section 14b, a transmission voltage amplitude controlling section 14c, and a resistor 14d. The video signal processing section 14b rearranges a video signal vi into a signal receivable by the data drivers 131, 132, . . . , 13M, and 13M+1. The transmission voltage amplitude controlling section 14c sets the amplitude of a transmission voltage based on a resistance value of the resistor 14d, to adjust the output voltage amplitude of a data signal vj which is output from the data signal output section 14a. The resistance value of the resistor 14d is determined by performing EMI evaluation. In the EMI evaluation, EMI properties are measured by changing the resistance value of the resistor 14d by trial and error, to determine such a resistance value that the EMI properties may be optimized.
In the present image display device, the video signal vi is input to the timing controller 14, in which it is processed using the video signal processing section 14b to generate an input signal va. The amplitude of the transmission voltage is set optimally in the transmission voltage amplitude controlling section 14c based on the resistance value of the resistor 14d, to give a data signal vj having the adjusted output voltage amplitude, which signal is then output from the data signal output section 14a. The data signal vj is transmitted via the video signal lines 15 to the data drivers 131, 132, . . . , 13M, and 13M+1 respectively. Then, an image corresponding to the video signal vi is displayed on the display panel.
However, the resistance value of the resistor 14d is determined on the basis of results of the EMI evaluation and the video signal line 15 transmits the data signal vj having an effective voltage amplitude that corresponds to the adjusted output voltage amplitude. That is, the ultimately adjusted output voltage amplitude becomes a fixed value irrespective of the input video signal vi. Despite this, the video signal vi input to the timing controller 14 comes in various types, so that as the video signal vi, that is, a display pattern varies, the degree of the influence from wiring crosstalk changes. That is, the video signal vi changes always and, correspondingly the effective voltage amplitude also changes momentarily. Therefore, if the output voltage amplitude is determined as a fixed value based on the resistance value of the resistor 14d beforehand, the effective voltage amplitude of the data signal vj transmitted via the video signal line 15 may not be an efficient value that responds to a change in video signal vi.
This causes the effective voltage amplitude in the present image display device to exceed the input amplitude specification value for the data drivers 131, 132, . . . , 13M, and 13M+1 more than necessary depending on the input video signal vi, thereby increasing dissipation power and EMI. Moreover, in the recent years, the size and the resolution of image display devices are ever-increasing. This progress in technology increases the number of video signal lines required and clearly swelling dissipation power and EMI, so that it is desirable to transmit the video signal having an efficient effective voltage amplitude through the video signal line.
Besides the present image display device, this type of related art may include, for example, a liquid crystal display (LCD) described in Japanese Patent Application Laid-open No. Hei11-174406 (hereinafter, referred to as the related-art Patent Document 1).
In the LCD, a driver integrated circuit (IC) drives a liquid crystal panel so that an image may be displayed on the panel. The IC includes an output circuit, which output circuit controls the output of a transfer clock signal and a display signal, which is a logical signal, so that the logical signal may be supplied to the driver IC for the purpose of driving image display. In particular, in the present IC, the output current performance or the rising edge or trailing edge properties of the output voltage of the output circuit can be changed from the outside. In this case, the properties are made variable by applying a predetermined voltage from the outside. Alternatively, the properties are made variable by connecting a resistor externally.
Further, in a data drive device in an LCD described in Japanese Patent Application Laid-open No. 2003-208134 (hereinafter, referred to as the related-art Patent Document 2), a timing controller is used to arrange pixel data pieces input from the outside, a voltage of which data is then stepped down with a resistance voltage divider and output to a plurality of data transmission lines. The data signal transmitted via the plurality of data transmission lines is stepped up to an original driving voltage with a level shift array and then converted into an analog pixel voltage signal with a data driver and supplied to a data line. This will reduce EMI.
However, the above-mentioned related arts have the following problems.
That is, the LCD described in the related-art Patent Document 1 cannot solve the above-mentioned problems because it does not take into account an influence from wiring crosstalk although the output current performance or the rising edge or trailing edge properties of the output voltage of the output circuit can be changed with an externally applied voltage or an externally connected resistor.
The data drive device described in the related-art Patent Document 2 has a problem in that its hardware configuration may be complicated because it needs level converting means such as a resistance voltage divider or a level shift array immediately on the upstream side of a data driver, which resistance voltage divider is used to step down a data voltage and which level shift array is used to step it up to its original driving voltage for the purpose of reducing EMI. Further, similar to the related-art Patent Document 1, it does not take into account a change in transmission voltage amplitude caused by wiring crosstalk and is considered to be easily affected by external noise.
In view of the above, the present invention has been developed, and it is an object of the present invention to provide an image display device and a video signal processing method used in the same that can avoid a transmission error due to wiring crosstalk when a video signal is being transmitted via a video signal line by utilizing a differential transmission system.